1. Field of Invention
This invention relates to an improved cylindrical dampener cover used on an offset printing machine for supplying water to a plate cylinder. More particularly, the invention relates to a dampener roll cover for use in an offset printing machine prepared by knitting or weaving a water shrinkable yarn and a hydophillic yarn as ground yarns in the same course into a cover.
2. Prior Art
Prior art dampener roll covers used to supply water to the plate cylinder were prepared by fabricating a cloth made by knitting or weaving cotton into a cylindrical shape, mounting it onto a dampener roll, and stretching it by pulling the opposite ends of the cylindrically shaped cloth before use. Difficulties are encountered in this technique because the covering fabric is apt to be loosened by centrifugal force during the rotation of the roll, causing the supply of water to the plate cylinder to become uneven due to uneven gathering of the fabric, or cause patterning of the fabric to take place. Patterning of a fabric may be defined as the imprinting of the dampener roll cover surface onto a fabric. Patterning is caused when pressure is exerted on the dampener roll so as to bring the cover on the roll into contact with the fabric. This leaves an impression characteristic of the knit construction of the dampener roll cover on the fabric, causing frequent interruptions of the printing operation in order to replace the covering fabric with a new cover or to try to restretch the old one. This eventually results in a reduction of the efficiency of the printing operation, adding additional costs of material and labor.
In U.S. Pat. No. 3,242,554, it is disclosed that dampener roll covers can be made from water-shrinkable fabrics. These fabrics are made into a cylindrical cover, mounted on a dampener roll and immersed in water to shrink the cover on the roll. However, these particular water-shrinkable fabrics must first be wet-stretched and dried prior to use. They have a limited degree of stretch, and then shrink by less than the original stretching when wet. This particular covering has drawbacks, because with the shrinkage, the surface of the cover becomes uneven and is not smooth, thus causing a patterning of the fabric.
Additionally, U.S. Pat. No. 3,926,701 discloses a covering fabric for a dampener roll, which is comprised of yarn capable of shrinking by the action of water. However, the shrinkable yarns used therein are employed as the ground yarn of the covering with hydrophilic yarns used as the pile yarn. In mounting the covering fabric of this invention onto a dampener roll, the roll covered with the fabric is immersed in water, often warm water, and the ground yarns of the covering fabric shrinks to closely fit the covering fabric to the dampener roll. This causes the pile yarn to gather together to form an uneven surface on the fabric. This uneven surface causes patterning on a fabric. In addition, because shrinkage occurs on the dampener cover in the lengthwise direction, when mounting, sufficient fabric must be used in order to accommodate the lengthwise shrinkage of the dampener cover to cover the dampener roll.
It is therefore difficult, if not impossible, to be able to calculate the correct length of the covering fabric that should be used. Thus, one must use an excess of material to insure an adequate fit. Furthermore there is a loss that arises with respect to the material and man hours that may be lost due to the material that may have to be removed after shrinkage of the covered fabric to the dampener roll.
The present invention has succeeded where others have failed in obtaining an excellent covering fabric for a dampener roll without the drawbacks encountered with either the conventional structured covers or by the prior art water-shrinkable covers.
An object of this invention is to provide a cover fabric that will eliminate the patterning of a product by a dampener roll cover.
Another object of this invention is to provide a covering fabric that has superior circumferential shrinkage and gripping action on the surface of a dampener roll.